I am using ESX 3.5i - not sure what you are using. I don't know of a way to do it per machine, but you can edit either the VM Port Group or the virtual switch properties and use traffic shaping to limit it for all machines in the port group or on the whole virtual switch. Click on the Host>Configuration tab>Networking>Properties for the Virtual Switch>Edit button for the switch or port group...>Traffic Shaping tab. You can create a new port group on the same vswitch or make a new vswitch and just put the one test machine on it so your other VMs aren't affected. I personally have never done it, but it looks like that would be the way to do it...

This will allow you to throttle the bandwidth to get it in the right range but it wont allow you to replicate the latency and other characteristics that have a significant impact on 56k modem "performance". To give a realistic emulation of the 56k modem experience you'll need to add in about 60ms of round trip latency. It's also worth noting that a 56k modem can deliver at most about 63kbps aggregate uplink and downlink and they never deliver better than 33.6kbps uplink.
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HelvickJul 10 '09 at 20:46

@August: Good point, I didn't mention I was using VMware Workstation.
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JoshJul 12 '09 at 12:54

@Helvick: I have options to limit the bandwidth of a LAN segment to modem speeds including generating packet loss. I wonder if this is also adding latency as you mentioned? Problem is, it only affects LAN segments, not the main ethernet connection.
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JoshJul 12 '09 at 12:57